A conventional automatic transmission to be mounted on a vehicle is adapted to transmit drive force from an engine to a transmission mechanism through a torque convertor, and constructed to engage or disengage clutches and brakes constituting a friction engagement element, and to a switch power transmission path formed by a plurality of gear pairs to form a shift range and a desired shift stage.
The clutches and brakes are operative to have their engagement state and disengagement state alternatively switched in accordance with hydraulic paths switched by operating various solenoid valves and a manual valve forming part of a hydraulic pressure circuit.
Here, the engine is operative to rotate an oil pump which in turn produces a line pressure to be supplied to each of hydraulic paths operative to be switched by the manual valve. Further, the line pressure to be supplied to the hydraulic paths are adjusted by the solenoid valves provided on the hydraulic paths at the downstream sides of the manual valve to be supplied to cylinders to operate the clutches and the brakes.
When the manual valve is operated, each of the shift ranges are formed, and concurrently the oil pressure to be supplied to the hydraulic cylinders is adjusted, thereby causing the clutches and the brakes to be switched to any one of the engagement and disengagement states.
The manual valve is operatively connected with a shift lever through a connection member, to ensure that the shift ranges are formed in response of the operation positions of the shift lever.
In recent years, there has so far been proposed a shift-by-wire system which is so called to electrically perform a transmission control in response to the operation positions of the shift lever. In response to the development of the shift-by-wire system, there appears an automatic transmission provided with no manual valve to be increased in number (for example see Patent Document 1).
The conventional automatic transmission disclosed in the Patent Document 1 is provided with no manual valve, so that any one or more solenoid valves is preliminarily set to be operated based on the electromagnet valve logic, thereby operating the solenoid valves corresponding to the desired backward advance shift stage and the forward advance shift stages. In this way, the conventional automatic transmission disclosed in the Patent Document 1 operates the solenoid valves corresponding to the desired shift stages, so that the brakes and the clutches can be operated on the electromagnet valve logic to form the desired backward advance shift stage and the forward advance shift stages.
Due to the fact that the automatic transmission can realize the changes of the shift stage and the transmission shifts by the shift-by-wire system but is provided with no manual valve, an ECU (Electronic Control Unit) is adapted to control the solenoid valves based on the detection results from a plurality of shift sensors provided at the operation positions of the shift lever and the electromagnet valve logic.